Disorders of water balance (i.e., hypernatremia and hyponatremia) are the most frequently encountered of the electrolyte problems; they are exceedingly common among the elderly, the acutely and chronically ill, and among patients taking certain diuretic and anti-depressant medications. All of these predisposing factors are present in a high percentage of patients served by the Department of Veterans Affairs. Aberrant water balance further complicates many chronic medical conditions and confers a poor prognosis independent of the severity of the underlying illness. Recent data from well-controlled studies indicate that even subtle abnormalities in systemic water balance can seriously affect coordination and cognition, and the propensity to falls. Regulation of body water balance is a complex process coordinated by the hypothalamus and executed by the kidney collecting duct; however, the central sensors of systemic tonicity had remained obscure. Recently, members of the transient receptor potential family of cation channels were proposed to serve this role, based upon extensive in vitro data and animal studies. We reasoned that a common single- nucleotide polymorphism in one of these genes may account for individual differences in systemic water balance among healthy elderly human subjects. We identified one such polymorphism in a key water- regulatory gene that confers an amino acid change and results in a protein with aberrant function in vitro. Importantly, we found that the presence of this polymorphism is strongly associated with hyponatremia in two healthy human populations. Furthermore, the effect size is large: the risk of hyponatremia is more than doubled by the presence of a single copy of this allele. To cement this relationship, we propose three experimental aims. In Aim I, we will sequence the haplotype block in which this polymorphism is embedded, to establish that no other polymorphism in tight linkage disequilibrium with our known variant accounts for the effect of this allele. Although we strongly suspect that this is the case, based upon our sequencing of all coding exons and splice junctions in this block, a robust conclusion requires phasing the entire haplotype block. This will also aid our investigation of multiple ethnicities. In Aim II, we will test additional large human populations for the presence of this polymorphism to confirm its association with serum sodium concentration and with hyponatremia. In Aim III, we will test for the role of epoxyeicosatrienoic acid signaling in systemic water balance in normal mice. Second, we will knock in the variant allele of the water-regulatory gene in a mouse model to test if it can independently recapitulate the human hyponatremic phenotype. We further propose to apply a number of physiological maneuvers to unmask a latent water-retentive phenotype. There is extremely high conservation between the human and mouse gene, including the amino acid affected by this polymorphism and its immediate context. Of note, we will not create a transgenic harboring an extra allele with this polymorphism; we will replace one or both wild-type alleles with a murine analog of this variant allele. In addition, in this Aim we will test the role of the epoxyeicosatrienoic acid system in regulating systemic water balance, in both wild-type and knock-in mice. There are no data addressing the role of this system in water balance in vivo, although it mediates tonicity-sensing by central osmosensing channels and we now show in our preliminary data that this effect is interrupted by our key polymorphism.